Workflow issue 1: main timeline and symbol timelines

I’m having some problem with the workflow of flash.

I make web banners everyday att my work att it is important to me
to have the banners easy structured so I can copy effects, tweens and symbols to
a new banner. I have Flash CS3 pro.

I always start by making a symbols. And then in the symbols own timeline I make the animation or effect.

The big problem I’m having is when I have the main (scene1) timeline with many frames, and then have symbols on that timeline, with it’s own timeline with some frames. The frames in the symbol timeline is okey, but at the main timeline the frames is too short or too long so the animation of the symbol cuts of early or it plays more than one time (too many frames).

How do I calculate how long a symbols timeline should be so It maches the main timeline?
What happens to me is that I always have to adjust the symbols timeline and the main timeline, and this I have to do many times.